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DALLAS (Reuters) - The outbreak of West Nile disease in the United States moved a step closer on Wednesday to becoming the second worst on record with federal health authorities reporting 280 cases of the virus-caused illness over the past week.

There have now been 4,249 cases of West Nile recorded this year, according to the Centers for Disease Control and Prevention, 20 cases fewer than in 2006, the second-largest outbreak on record.

The number of deaths rose by five to 168 since last week, the CDC said.

The worst year on record for West Nile disease was 2003, when 9,862 cases were reported, the CDC said.

The pace of new cases of the disease - which is transmitted from infected birds to humans by mosquitoes -- has slowed since late summer, authorities said.

More than 70 percent of the cases have been reported in eight states: Texas, Mississippi, Michigan, South Dakota, Louisiana, Oklahoma, Illinois and California. Texas has been the hardest hit, recording close to 40 percent of the cases in the country, according to the CDC.

The Dallas-Fort Worth area has been the epicenter of this year's outbreak, with 33 deaths, according to the Texas Department of State Health Services.

The Dallas Morning News reported Wednesday that health officials in Tarrant County, home to Fort Worth, may have vastly underreported cases of the serious neuroinvasive form of West Nile.

The report said the number of cases detected through Tarrant County blood bank screenings - information that helps public officials determine the size of an outbreak - differs significantly from what officials there have reported.

But state health officials said it was unlikely cases were overlooked.

The severe neuroinvasive form of the disease almost always requires hospitalization and can lead to meningitis, encephalitis and death, according to the CDC.

"Neuroinvasive disease is not subtle," said Carrie Williams, a spokeswoman for the Texas health department. "We're confident physicians knew about the outbreak and were on the lookout for disease."

Nationwide, half of cases reported to the CDC have been of the neuroinvasive form. The other half are West Nile Fever, a milder form which causes flu-like symptoms and is not deadly.

Outbreaks tend to be unpredictable and are typically triggered by a combination of hot weather and intermittent rainfall as well as ecological factors such as the size of the bird and mosquito populations.